Activation of the basolateral membrane Cl − conductance essential for electrogenic K + secretion suppresses electrogenic Cl − secretion

Adrenaline activates transient Cl − secretion and sustained K + secretion across isolated distal colonic mucosa of guinea‐pigs. The Ca 2+ ‐activated Cl − channel inhibitor CaCCinh‐A01 (30 μ m ) significantly reduced electrogenic K + secretion, detected as short‐circuit current ( I sc ). This inhibition supported the cell model for K + secretion in which basolateral membrane Cl − channels provide an exit pathway for Cl − entering the cell via Na + –K + –2Cl − cotransporters. CaCCinh‐A01 inhibited both I sc and transepithelial conductance in a concentration‐dependent manner (IC 50 = 6.3 μ m) . Another Cl − channel inhibitor, GlyH‐101, also reduced sustained adrenaline‐activated I sc (IC 50 = 9.4 μ m ). Adrenaline activated whole‐cell Cl − current in isolated intact colonic crypts, confirmed by ion substitution. This adrenaline‐activated whole‐cell Cl − current was also inhibited by CaCCinh‐A01 or GlyH‐101. In contrast to K + secretion, CaCCinh‐A01 augmented the electrogenic Cl − secretion activated by adrenaline as well as that activated by prostaglandin E 2 . Synergistic Cl − secretion activated by cholinergic/prostaglandin E 2 stimulation was insensitive to CaCCinh‐A01. Colonic expression of the Ca 2+ ‐activated Cl − channel protein Tmem16A was supported by RT‐PCR detection of Tmem16A mRNA, by immunoblot with a Tmem16A antibody, and by detection of immunofluorescence in lateral membranes of epithelial cells. Alternative splices of Tmem16A were detected for exons that are involved in channel activation. Inhibition of K + secretion and augmentation of Cl − secretion by CaCCinh‐A01 support a common colonic cell model for these two ion secretory processes, such that activation of basolateral membrane Cl − channels contributes to the production of electrogenic K + secretion and limits the rate of Cl − secretion. Maximal physiological Cl − secretion occurs only for synergistic activation mechanisms that close these basolateral membrane Cl − channels.